compiler: always initialize mpfr in integer import

Test case is https://go.dev/cl/413980.

Fixes golang/go#52862

Change-Id: I2233878df0ee075a5cc60e32151cc9802e9b7075
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/413981
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/go/expressions.cc b/go/expressions.cc
index 734ecb9..135dae0 100644
--- a/go/expressions.cc
+++ b/go/expressions.cc
@@ -2715,7 +2715,7 @@
 	  return Expression::make_error(loc);
 	}
       if (pos == std::string::npos)
-	mpfr_set_ui(real, 0, MPFR_RNDN);
+	mpfr_init_set_ui(real, 0, MPFR_RNDN);
       else
 	{
 	  std::string real_str = num.substr(0, pos);